One method is wedges and shims also commonly called wedges and feathers. I'm not advocating that company but the pic shows exactly what they are.

This is a pretty good video on how to use the wedges and feathers.

Watch this youtube video. Link below.

Here is a link that might be useful: granite splitting

Feathers and wedges. Drill holes every 3-6 inches depending on thickness and stone, as deep as possible. Insert feathers and snug in place with wedges in a line where the outward force of the wedge corresponds to the direction of cleavage. Tap wedges with a hammer in linear progression. Best to start away from edge and move back and forth. You'll hear it go! Where are you in Maine, I could do it for you if you are near S. Maine, N.H. Border.

As a cool anecdote, somebody who works with old buildings once told me that the

1) drill the holes - has been around a while
but the

2) feathers and wedges - are pretty recent.

This guy told me that way back then, the stones were drilled, then left in the weather for a few winter seasons. The ice damage would replace the work of the feathers and wedges.

Back then, seems folks had more time than tools